In recent years, electricity storage devices that are used for portable information terminals, electric vehicles, power storage apparatuses for household use, and the like have been progressively developed. Among electricity storage devices, capacitors and nonaqueous electrolyte secondary batteries have been actively studied. In particular, as electricity storage devices that are capable of increasing capacity and energy density and have a high degree of safety, expectations are high for development of molten salt batteries. Molten salt batteries use incombustible molten salt electrolytes and, therefore, are safer than electricity storage devices that use nonaqueous electrolytes, e.g., lithium-ion secondary batteries.
A molten salt battery includes an electrode group including a first electrode, a second electrode, and a separator interposed therebetween; and a molten salt electrolyte. Each electrode includes a current collector (electrode core) and an active material disposed on the current collector. The electrode group and the molten salt electrolyte are, for example, housed in a rectangular case. In an electricity storage device having a rectangular case, it is common to place a cover plate on the opening of the case such that electricity-generating elements, such as an electrode group and a molten salt electrolyte, are hermetically enclosed in the case (refer to PTL 1).
On the other hand, with increases in the capacity and volume energy density of electricity storage devices, there is an increased need for provision of a mechanism for ensuring the safety of electricity storage devices. One mechanism for ensuring the safety of electricity storage devices is a safety mechanism in which a thermal fuse is disposed in a current path (refer to PTL 2). When the battery temperature rises abnormally, the safety mechanism operates to shut off a charging/discharging current.